Modulation of Ribosomal RNA Synthesis in Oncopeltus fasciatus: An Electron Microscopic Study of the Relationship between Changes in Chromatin Structure and Transcriptional Activity

I have used an electron microscopic approach to study transcriptional control of ribosomal RNA (rRNA) synthesis in the milkweed bug Oncopeltus fasciatus (Dallas). This insect was chosen for investigation because biochemical studies indicated that the rate of rRNA synthesis is extensively modulated during embryonic development. Harris and Forrest (1967) determined, for O. fasciatus embryos of various developmental stages, both the absolute amount of rRNA per embryo and the relative rates of incorporation of [3H]uridine into RNA molecules that sediment at the speed of rRNA. They detected no rRNA synthesis in embryos 20 hours old (syncytial blastoderm stage; Butt 1949). Incorporation of [3H]uridine into rRNA was found to occur at low levels in 44-hour-old embryos (mid-germ band stage) and then to increase dramatically, reaching a peak at 68 hours of development (during neurulation). Afterwards, rRNA synthesis declines and is barely detectable in embryos more than 116 hours old. The data of...